1. Field of the Invention
This present invention relates to image forming apparatuses and manufacturing processes for producing ejection heads, and more particularly, relates to image forming apparatuses with a liquid ejection head using PZT actuator and to manufacturing processes and system for production of an liquid ejection head with uniform liquid injection from different nozzles.
2. Description of the Related Art
Image forming apparatus such as a printer, a copier and a facsimile, and also a multi function type device combining these machine's functions are generally known. The image forming apparatus, for example, has at least one recording head ejecting droplets of ink and forming images by attaching the ink to a recording or transfer medium such as for example a recording sheet or paper being transferred in the image forming apparatus. The term of “medium” as used herein refers to “paper” as generally used in this field but is not limited to merely paper but refers also to any material, or recording medium, or transfer material, or recording sheet, and so on.
Additionally, the term “image forming apparatus” as used herein and as generally used in this field refers to a device discharging a liquid into a medium, such as paper, thread/string, fiber, textiles, leather, metal, plastic, glass, wood/lumber, ceramics, and the like. Further, the term “image forming” as used herein and as generally used in this field refers to an image being formed onto a medium, such as the forming of letters, characters, figures, and transferred patterns, and so on one or more of the mediums listed above.
The liquid ejection head as used herein and as generally used in this filed refers to piezoelectric type heads using piezoelectric actuators. In particular, piezoelectric type heads use a plurality of piezoelectric devices which can pressure ink in a reservoir communicating with a corresponding nozzle, can deform a member (or diaphragm) on one side of the surface of the reservoir (capable of elastic deformation), and can eject the liquid (or ink) by changing a volume and a pressure in each reservoir.
One image forming apparatus using the above described liquid ejection head as used herein and as generally used in this field is a line-type image forming apparatus having a line-type ejection head which can have a plurality of nozzles aligned along a paper edge entirely, and can record images onto a medium which is transferred to an orthogonal direction against the paper width at high-speed and on one-pass without necessarily using head scanning.
Such a line-type image forming apparatus can produce high quality images at high speed and high reliability. So, the uniformity of droplet ejection characteristics between each nozzle of the line-type ejection head is an important criterion. For this reason, a head, which exhibits a narrow range of droplet ejection speed and volume variation of droplets, is considered desirable.
However, it is known that a piezoelectric actuator for a high ejection frequency nozzle is prone to change its characteristics more readily than that for a lower frequency nozzle. It is also known that the characteristics of a piezoelectric actuator can gradually change over time because of changes of the environmental temperature around the actuator. Therefore, the droplet ejection characteristics between each nozzle of the recording head can gradually vary over time, so the recorded image quality can degrade.
Japanese Unexamined Patent Application Publication No. 10-193601 describes an ink jet recording apparatus that is equipped with a repolarization device which attempts to recover the characteristics of piezoelectric actuators by repolarizing the piezoelectric actuators after use. However, the repolarizing all nozzles through the application of a constant polarization voltage sometimes does not recover a variation of droplet ejection characteristics sufficiently.
Japanese Unexamined Patent Application Publication No. 2001-277525 describes a polarization adjustment method for piezoelectric actuators that adjusts a polarization degree of a piezoelectric actuator of each nozzle of a recording head. This method improves a variation of droplet ejection speeds or droplet volumes between nozzles. However, Japanese Unexamined Patent Application Publication No. 2001-277525 describes that this method applies to a head manufacturing apparatus. Moreover, this method adjusts the polarization degree with respect to each nozzle one by one, in such a way that a polarization adjustment of a plurality of processes is completed in response to one of adjustment target nozzles, and next, in response to another.